Digital-to-analogue converter using an array of current sources

ABSTRACT

A current source DAC has calibration of the current sources used for providing the analog output. There are two outputs, one of which provides the output current or else a differential output is provided. The calibration is cyclic and the current source outputs switched to the output terminals are selected as a function of the point within the calibration cycle. The current stage of the cyclic calibration process is thus taken into account in the D/A conversion. For example, the average time since calibration for all current sources having outputs switched to the first output may be approximately equal to the average time since calibration for all current sources having outputs switched to the second output. In this way, the average current of the cells switched to one terminal is identical to the average current of the cells switched to the other terminal, and the average current of the cells switched to each terminal remains constant in time irrespective of the digital signal value being converted. This ensures DAC linearity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to digital-to-analogue converters, andparticularly to converters which use an array of current sources toprovide an analogue output.

2. Related Art

Digital-to-analogue converters of this type are widely used in numerousapplications, and can be implemented as high speed converters suitablefor use in telecommunication systems. In one arrangement, the array ofcurrent sources is arranged as a plurality of identical current sources.An associated switching array selectively routes the current sourceoutputs to two possible output terminals. In some systems, one of theseterminals is simply ground, and the other terminal is a current summingbus. The signal on the current summing bus is converted into a voltagebefore being provided as an output of the converter. In other systems,the output of the converter is to be provided on to twisted paircabling. In this case, the current provided to one of the outputterminals is converted to a voltage for one of the twisted pair wires,and the current provided to the other terminal is converted to a voltagefor the other wire of the twisted pair. In this case, the converteroutput is represented as the difference between the voltage levels onthe two wires of the twisted pair.

It has been recognised that to provide high precision, for example whenconverting more than 10 bits, it is necessary to calibrate the currentsources constantly during the conversion process.

U.S. Pat. No. 5,870,044 discloses one circuit and method for providingcalibration of the current sources used in the converter array.Essentially, an additional current source is provided, and the output ofthe additional current source is used to replace a current source withinthe array when that particular current source within the array is beingcalibrated. In this way, all current sources within the array can becalibrated in a cyclical manner. The additional current source avoidsthe need to interrupt the conversion process during calibration.

The calibration of individual current sources enables compensation forvariations in layer thicknesses or other dimensions across the currentsource array. These variations across the array are static errors. Thereare also dynamic errors which result from the various parasiticcapacitances within the current source circuits. After calibration of acurrent source to provide a desired output, changes in the chargesstored on these parasitic capacitances vary the current source outputover time. There is therefore also a need to carry out a continuouscyclical calibration process.

SUMMARY OF THE INVENTION

The invention is based on the recognition that a significant portion ofthe error in the output current of each current source will be afunction of the time since that particular current source wascalibrated. In particular, it has been recognised that there is asubstantially linear increase in the error in the output current foreach current source as a function of time since the most recentcalibration of that current source.

According to the invention, there is provided a digital-to-analogueconverter, comprising:

an array of current sources, each current source having an output whichis switchable onto one of first and second terminals as a function of adigital input, such that a first number of current source outputs areswitched to the first terminal and a second number of current sourceoutputs are switched to the second terminal,

wherein the current source outputs are also switchable to a calibrationunit, the current sources of the array being calibrated cyclically,

wherein the current source outputs switched to the first and secondoutput terminals are selected as a function of the point within thecalibration cycle.

One terminal may be an output and the other terminal may be ground.Alternatively, the two outputs may define a differential output signal.

The operation of the converter of the invention takes into account thecurrent stage of the cyclic calibration process. For example, theaverage time since calibration for all current sources having outputsswitched to the first terminal may be approximately equal to the averagetime since calibration for all current sources having outputs switchedto the second terminal. In other words, the average time sincecalibration for each terminal is equal to half the calibration cycleduration.

By making the average time since calibration equal for the currentsources switched to the two terminals, the average current of the cellsswitched to one terminal is identical to the average current of thecells switched to the other terminal, because the average current erroris the same. The average current of the cells switched to each terminalremains constant in time and is independent of the digital input beingconverted. This ensures DAC linearity, both for single and double outputsystems.

In order to equal the average time since calibration for the currentsources switched to the two terminals, the current sources most recentlycalibrated and the current sources calibrated the longest time ago maybe switched to one of the output terminals, with the remaining currentsources switched to the other output terminal.

The array of current sources may comprise a first plurality of identicalcurrent sources, the outputs of which are switched in dependence on themost significant bits of the digital input, and a second plurality ofcurrent sources, the outputs of which are switched in dependence on theleast significant bits of the digital input.

This arrangement enables the advantages of identical current sourceswith the advantages of a binary array of current sources to be combined.In particular, the second plurality of current sources can comprise twocurrent sources, and wherein the output of each current source of thesecond plurality is provided with a divider arrangement for providingthe output on a plurality of lines, the currents on the individual linesbeing switchable to the first or second output terminals under thecontrol of the least significant bits of the input.

This converter arrangement may implement a 14 bit converter, with 128current sources for the seven most significant bits, and two currentsources for the seven least significant bits.

The invention also provides a method of controlling adigital-to-analogue converter, the converter comprising an array ofcurrent sources, each current source having an output which isswitchable onto one of first and second terminals as a function of adigital input, such that a first number of current source outputs areswitched to the first terminal and a second number of current sourceoutputs are switched to the second terminal, the method comprising:

cyclically calibrating the current sources of the array;

switching a first subset of the current source outputs to the firstterminal and switching a second subset of the current source outputs tothe second terminal, the first and second subsets together including allcurrent sources other than the or each current source being calibrated,the switching being in dependence on the input digital signal,

wherein the first and second subsets are selected as a function of thepoint within the calibration cycle.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

An example of the invention will now be described in detail withreference to the accompanying drawings.

FIG. 1 shows a digital-to-analogue converter of the invention, and whichis controlled in accordance with the method of the invention.

FIG. 2 is used to explain the calibration of individual current sources.

FIG. 3 is used to explain the operation according to the invention.

FIG. 4 shows in greater detail a preferred converter architecture of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a converter of the invention using an array of currentsources S. For the N most significant bits of the digital word to beconverted, an array of n identical current sources S₁ to S_(n) isprovided, where n=2^(N). The outputs of these n current sources areswitched to one of two outputs I_(out1) and I_(out2). The currentsflowing to these two outputs are converted to voltages and may then beprovided on output lines of the converter. The difference between thesignals on the two lines provides the analogue output. Alternatively,one of the two outputs can be ground, and the output of the converter isthen provided on a single line.

Only 2^(N)−1 of these current sources are required to represent the Nbits, and the additional n^(th) current source is an additional currentsource used when one of the 2^(N)−1 current sources is being calibrated.

FIG. 1 also shows a current source S_(n+1), the output of which isprovided to a divider 10, which provides a number of outputs k derivedfrom the single current source S_(n+1). This provides a compact way ofconverting the least significant bits of the digital input. Each of thek outputs of the divider 10 can also be switched selectively to eitherof the two outputs I_(Out1) and I_(out2).

A second additional current source S_(n+2) is used to provide thecurrent derived from the LSBs when the current source S_(n+1) is beingcalibrated. This second additional current source is provided with adivider circuit 11.

The arrangement of FIG. 1 essentially therefore provides an array ofcurrent sources, with each current source being switchable on to one offirst and second output terminals as a function of the digital input. Inthis way, a first number of current source outputs are switched to thefirst output terminal and a second number of current source outputs areswitched to the second output terminal.

The current sources must be calibrated, as mentioned above. Inparticular, the output current from a current source will vary over timeas a result of leakage currents flowing within the circuitry of thecurrent source. It has therefore been recognised that the currentsources should be calibrated cyclically to maintain the desired accuracyof the conversion. For this purpose, the additional current source S_(n)can be used to replace the output of any other current source S₁ toS_(n−1) when that particular current source is being calibrated. Theadditional current source S_(n+2) can be used to replace the output ofcurrent source S_(n+1) when that current source is being calibrated. Thecurrent source outputs are switchable to a calibration unit (not shownin FIG. 1), and this is performed cyclically for all current sources ofthe array.

FIG. 2 shows one way in which an individual current source can becalibrated. Each current source may be arranged as first and secondcurrent source transistors 12, 14. The relative dimensions of the twotransistors 12, 14 are such that the first transistor 12 provides a muchsmaller output current, and the majority of the output current of thecurrent source is provided by the second transistor 14. For example, thefirst transistor 12 may provide {fraction (1/32)} of the output currentof the current source. This smaller transistor 12 has a gate voltagewhich is controlled to provide adjustment of the total current output ofthe current source. The gate voltage on the second transistor 14 isfixed, and is common for all current sources at V_(G(com)). Duringcalibration of the current source, the output is switched by transistor16 to a reference current source 18, which forces the output of thecurrent source to the desired value. A feedback control 20 provides anappropriate gate voltage on the transistor 12 for the current source toprovide the desired output current. Switch 22 is closed during thecalibration process, and is opened at the end of the calibrationprocess, and the required gate control voltage is held on the gate oftransistor 12 by the MOS gate capacitance.

After calibration of the current source, the leakage current of theswitch 22 discharges the control gate voltage of transistor 12. Thisresults in a change in the output current of the current source. This ispredominantly a first order effect, which means that the resulting errorin the output of the current source varies in a linear manner over time.The invention provides a method of operating a digital-to-analogueconverter, such as that shown in FIG. 1, in such a way as to takeadvantage of this linear dependency of the output error of each currentsource on the time since calibration.

In accordance with the invention, the current source outputs which areswitched to the two output terminals are selected as a function of thepoint within the calibration cycle. This enables the average time sincecalibration for all current sources switched to the first and secondoutput terminals to be the same.

FIG. 3 is used to explain one way of implementing the method of theinvention to achieve equal average time since calibration.

FIG. 3A shows the error in the output of each current source S₁ to S₁₂₈.This assumes there are seven most significant bits. The 128 currentsources include the 127 current sources required for converting theseven most significant bits, as well as an additional current sourcerequired to enable simultaneous calibration to take place.

In the example of FIG. 3A, current source number 1 is assumed to be inthe process of calibration, and the error in the output is thereforerepresented as 0. FIG. 3 assumes the current sources are calibrated innumerical order. As seen from FIG. 3A, current source 2 has the largesterror, since the time since that current source was previouslycalibrated is the largest. There is a linear decrease in the error fromcurrent source 2 through to current source 128, which was calibratedduring the previous calibration cycle. In FIG. 3, the hatched areas 30indicate the current sources, the outputs of which are switched to oneof the terminals, and the non-hatched area 32 represents the currentsources, the outputs of which are switched to the other output terminal.

If we assume the digital input requires a number p of current sources tobe switched to the first output terminal, half of this output will beprovided by current sources 2 to (1+p/2) and the other half will beprovided by current sources (128−p/2) to 128. In other words, thecurrent sources most recently calibrated and the current sourcescalibrated the longest time ago are switched to one of the outputterminals, and the intermediate current sources are switched to theother output terminal. The result is that the average time sincecalibration for all current sources switched to the first outputterminal is the same as the average time since calibration for allcurrent sources switched to the second output terminal.

Since the average time from calibration for all current cells switchedto one terminal remains constant, this approach improves the linearity,regardless of whether a two terminal differential output is provided ora single terminal output.

FIG. 3B shows the same technique as described with reference to FIG. 3A,when the current source being calibrated is number 47.

Although the method described with reference to FIG. 3 takes the mostrecently calibrated current sources and the current sources calibratedthe longest time ago, other schemes exist for ensuring that the averageoutput errors remain the same.

The effect of ensuring substantially equal average errors of the currentsources switched to each output is that the error is constant. It istherefore possible to assume that all current cells have the same outputcurrent value, which may be slightly different to the ideal level. Thisdifference results in slightly modified converter gain, but thelinearity is preserved. This means the same step between successiveoutputs is provided with suppressed harmonics and low noise. Thislinearity can be more important than the absolute gain.

FIG. 4 shows one specific example of digital-to-analogue converteraccording to the invention. This specific example is arranged as a 14bit converter. For the seven most significant bits, an array of 128current sources S₁ to S₁₂₈ are provided. 127 of these are in use at anyone time, and one is being calibrated. As described with reference toFIG. 2, calibration involves applying a tuning voltage V_(TUNE). Theoutput of each of these MSB current sources is provided to a three wayswitch 40 which enables the current source output to be routed to thepositive output 42, the negative output 44 or the calibration unit 46. Asingle current source LSB1 is provided for generating outputs dependentupon the seven least significant bits. This current source LSB1 has adivider 10 at its output. This comprises an array of transistors with acommon gate voltage V_(B), and the width to length ratio of thetransistor channel determines the source-drain current flowing througheach transistor. The array of transistors is arranged as 31 transistorseach providing an output which is {fraction (1/32)} of the total currentsource output, one transistor providing an output of {fraction (1/64)}and two outputs each providing {fraction (1/128)} of the current sourceoutput. One of these {fraction (1/128)} outputs is used only duringcalibration of the current source and is switched to ground 47 duringthe use of the current source LSB1. All other outputs are switched tothe positive or negative output terminals 42,44 during the use of thecurrent source LSB1 and to the calibration unit during calibration.During calibration, all outputs are thus switched to the calibrationunit and the combined output current should then be equal to the outputcurrent of one of the MSB current sources. This enables the samecalibration unit to be used for calibrating the MSB and LSB currentsources.

The combined currents flowing to the two output terminals are convertedto voltages by resistors 48. During calibration of the current sourceLSB1, an additional current source is required providing the same scaledoutputs. For this purpose, the second current source for the leastsignificant bits is provided. This is shown only schematically as LSB0in FIG. 4 but corresponds to current source S_(n+2) and divider 11 inFIG. 1.

Although not shown in FIG. 1 or 4, additional switches are required toprevent the output current from a current source from flowing to thepositive or negative outputs when the current source is beingcalibrated.

The calibration circuit 46 essentially comprises additional currentsources providing a reference output to which the current sources of theconverter are tuned. This reference output may be derived from a numberof additional current sources distributed around the substrate of theconverter. This ensures that the reference output is within the range ofeach individual current source that can be achieved by controlling thetransistor 12, shown in FIG. 2. The full array of current sources may bearranged as a two-dimensional array, for example with 13 columns and 12rows. For the 14 bit converter of FIG. 4, this provides 16 currentsource circuits in addition to the 130 required by the converter. Someor all of these 16 additional current sources are used for biasing andreference purposes, in conventional manner.

The method of the invention enables a significant improvement in thesignal to noise ratio at the output of the converter, particularly thesignal to noise ratio of the current output. The signal to noise ratioof the voltage output of the converter has increased noise as a resultof the temperature dependence of the resistors 48 used for conversion ofthe current output to a voltage.

One preferred calibration cycle involves calibrating half of the MSBcurrent sources (for example current sources I₁ to I₆₄), calibrating oneof the LSB current sources, calibrating the remaining MSB currentsources (for example I₆₅ to I₁₂₈) and finally calibrating the other LSBcurrent source. By way of example, the converter may be running with aclock speed of 17.664 MHz, and one clock cycle is required for a D/Aconversion. Each calibration operation may occupy 256 clock cycles (14.5μs) which gives a total calibration time for the 130 current sources of1.88 ms. This corresponds to a calibration frequency of 531 Hz.

The invention provides a scheme by which the current sources areselected based on the time since calibration. In the specific exampleabove, the average time since calibration is selected to be equal sothat for all outputs of the DAC, the current source error can beconsidered to be a constant value, thereby preserving linearity of theconverter. It is also possible to select the current sources accordingto different criteria. For example, it is possible to select the currentsource outputs such that the total error in the summed current providedto the two outputs is as close as possible to equal. In this case, thetotal time since calibration for all current sources having outputsswitched to the first output terminal is as close as possible to thetotal time since calibration for all current sources having outputsswitched to the second output terminal.

The invention enables the effect of switch leakage currents to bereduced or cancelled. Higher conversion accuracy can thus be achieved.This increased accuracy may be used to enable the calibration frequencyto be reduced, and also enables a reduction in the required MOS gatecapacitance, as the charge leakage resulting from a reduced gatecapacitance is compensated. The selection of cells based on thecalibration cycle also means that different current cells are used forthe same digital input depending on the prevailing stage within thecalibration cycle. This has a further averaging effect which improvesthe converter linearity.

Further advantages and features of the invention will be apparent tothose skilled in the art.

What is claimed is:
 1. A digital-to-analogue converter, comprising: anarray of current sources, each of said current sources having an outputthat is switchable onto one of first and second output terminals as afunction of a digital input signal, wherein a first group of currentsource outputs are switched to the first output terminal regardless ofbeing a most significant bits (MSB) or a least significant bits (LSB)current source and a second group of current source outputs are switchedto the second output terminal regardless of being a MSB or LSB currentsource; and a calibration unit, wherein any of said current sourceoutputs are also switchable to said calibration unit, wherein thecurrent sources are calibrated cyclically during a calibration cycle,and wherein the current source outputs switched to the first and secondoutput terminals are selected as a function of a point within thecalibration cycle.
 2. The converter as claimed in claim 1, wherein anaverage time since calibration for said first group of said currentsources is approximately equal to an average time since calibration forsaid second group of said current sources.
 3. The converter as claimedin claim 2, wherein said first group of current source outputs is thecurrent sources most recently calibrated and least recently calibratedand said second group of said current source outputs is a remaining setof the current sources, whereby an average time since calibration forall said current sources switched to said first output terminal is equalto an average time since said calibration for all said current sourcesswitched to said second output terminal.
 4. The converter as claimed inclaim 1, wherein the array of current sources comprises said MSB currentsources, the outputs of which are switched in dependence on the mostsignificant bits of the digital input, and said LSB current sources, theoutputs of which are switched in dependence on the least significantbits of the digital input.
 5. The converter as claimed in claim 4,wherein the LSB current sources comprises first and second currentsources, and wherein the output of said first and second current sourcesis divided to produce currents on a plurality of lines, the currents onthe individual lines being switchable to the first or second outputterminals under control of the least significant bits of the digitalinput signal.
 6. The converter as claimed in claim 1, wherein eachcurrent source comprises: a first current source transistor comprising agate voltage controlled to provide adjustment of a total current outputof the current source; a second current source transistor comprising afixed gate voltage, wherein the first current source transistor providesa fraction of the total current output current of the current source andthe second current source transistor provides a majority of the totalcurrent output current of the current source; a reference currentsource; and a third current source transistor switching the totalcurrent output of the current source to the reference current sourceduring calibration of the current source.
 7. The converter as claimed inclaim 6, further comprising: a feedback control providing a gate voltageto the first current source transistor so that the current sourceprovides a desired output current.
 8. The converter as claimed in claim1, wherein the calibration unit compares the output of each said currentsource with a reference source.
 9. The converter as claimed in claim 1,wherein 14 bits are converted and wherein the array comprises 128 ofsaid MSB current sources corresponding to 7 most significant bits andtwo of said LSB current sources corresponding to 7 least significantbits.
 10. The converter as claimed in claim 1, wherein selecting thepoint within the calibration cycle enables an average time sincecalibration for all current sources switched to the first and secondoutput terminals to be equal.
 11. A digital-to-analogue converter,comprising: an array of current sources, each said current source havingan output that is switchable onto one of first and second outputterminals as a function of a digital input signal, wherein a first groupof current source outputs are switched to the first output terminalregardless of being a MSB or LSB current source and a second group ofcurrent source outputs are switched to the second output terminalregardless of being a MSB or LSB current source; and a calibration unitthat calibrates the current sources, wherein the current source outputsare also switchable to the calibration unit, wherein the current sourcesare calibrated cyclically, and wherein the current source outputsswitched to the first and second output terminals are selected such thatan average time since calibration for said first group is approximatelyequal to an average time since said calibration for said second group.12. A method of controlling a digital-to-analogue converter, theconverter comprising an array of current sources, each of the currentsources having an output that is switchable onto one of first and secondoutput terminals as a function of a digital input signal, such that afirst number of current source outputs are switched to the first outputterminal and a second number of current source outputs are switched tothe second output terminal, the method comprising the steps of:cyclically calibrating the current sources; switching a first subset ofthe current source outputs to the first terminal regardless of being aMSB or LSB current source; switching a second subset of the currentsource outputs to the second output terminal regardless of being a MSBor LSB current source; basing the switching on the digital input signal;and selecting the first and second subsets as a function of a pointwithin a calibration cycle.
 13. The method as claimed in claim 12,further comprising the step of setting an average time since calibrationfor all said current sources in the first and second subsets as beingapproximately equal.
 14. The method as claimed in claim 13, furthercomprising the steps of: forming said first subset from the currentsources most recently and least recently calibrated; and forming saidsecond subset from remaining ones of said current sources, excluding atleast one of the current sources being calibrated.